The description is preceded by an explanation of a prior art control circuit illustrated in FIG. 1 by a block circuit diagram. As shown in FIG. 1, among various components connected in sequence to an FM IF detection input 8, reference numeral 1 denotes a high-pass filter (HPF), reference numerals 2, 3, 21 and 22 designate amplifiers, 4 refers to a white noise level detection circuit, 5 refers to an AGC drive circuit, 6 is a blend control circuit, 7 is a stereo demodulation circuit, 16 is a smoothing circuit, 17 is a 38 kHz switching pulse circuit, 18 is a detection circuit, 8 through 10 are terminals, and 11 and 12 are resistors. The circuitry of FIG. 1 is employed in an FM stereo demodulation circuit as a control circuit to improve the signal-to-noise ratio (SNR) of signals with intermediate and weak electric fields. Namely, the control circuit is configured to detect white noise levels in detection signals supplied from the FM IF stage and to vary the stereo channels separation of outputs from the FM stereo demodulation circuit between the maximum value and zero, i.e. monoral state, responsively to the detected white noise level. The high-pass filter 1, amplifier 2 and resistors 11 and 12 form an active high-pass filter so that the high frequency components such as white noise in the FM IF detection outputs entered in the terminal 8 pass through the control circuit but the FM composite signal components do not. The latter signal components pass through a low-pass filter 19 in FIG. 1, and pilot signals thereof are entered in the switching pulse circuit 17 via a band-pass filter 20 and an amplifier 21 whereas the composite signals are entered in the stereo demodulation circuit 7 via an amplifier 22. The white noise components extracted from the FM IF detection output by the said active high-pass filter are entered in the white noise level detection circuit 4 via the amplifier 3. If the detection circuit 4 detects white noise components above a given reference level, it supplies an electric current responsive to the magnitude of the detected white noise level. The output from the detection circuit 4 is smoothed and voltage converted by the smoothing circuit 16 to serve as control voltages of the AGC drive circuit 5 and of the blend control circuit 6. More specifically, when the voltage of the output from the smoothing circuit 16 is increased, the AGC drive circuit 5 controls the amplifier 3 to decrease the gain thereof. The blend control circuit 6 controls the 38 kHz switching pulse circuit 17 to decrease the amplitude of the output thereof for switching the stereo demodulation circuit 7. As the output amplitude of the 38 kHz switching pulse circuit 17 is decreased, the separation of the output from the stereo demodulation circuit 7 is varied from the maximum value to zero, i.e. monoral condition. More specifically, in the circuit of FIG. 1, an increase of the white noise level in the FM IF detection output entered in the terminal 8 invites a greater gain decrease of the amplifier 3 and a less separation into two output terminals 34 and 35 of the stereo demodulation circuit 7. FIG. 2 shows the gain attenuation of the amplifier 3 and the separation of the outputs from the stereo demodulation circuit 7 with respect to the white noise level. This shows that, in the prior art control circuit of FIG. 1, if the respective components or stages of the control circuit are given fixed constants, changes in the gain decrease of the amplifier 3 and in the separation of the output of the stereo demodulation circuit 7 are uniformly fixed. If it is required to change the point whereat the output separation of the stereo demodulation circuit 7 begins to vary with a change in the white noise level, the requirement is met by a change, for example, in the sensitivity of the white noise level detection circuit 4. However, this invites a change of the point whereat the automatic gain control of the amplifier 3 is commenced, followed by a change of the gain attenuation ratio of the amplifier 3. It should be noted here that the detection circuit 18 is also supplied with the output from the amplifier 3 to control other network, for example, other than the output separation of the stereo demodulation circuit 7. Therefore, a change in the gain attenuation of the amplifier 3 invites a problem that the point whereat the detection circuit 18 is rendered operative also varies. Additionally, in case that the control circuit is integrated on a semiconductor substrate, additional terminals unique to the control circuit must be provided.